1. Field of the Invention
The present invention relates to the field of hole cutting devices and specifically to plunge cutting devices capable of cutting a non-circular opening in a single operation.
2. Background Information
There has long been a need to cut non-circular holes rapidly and precisely. Applications in need of this capability include installing electrical boxes in drywall; cutting mortises for timber frame construction and similar woodworking joinery; and cutting outlines for craftwork, cutting boards, etc. The most common approach is to use a saw, such as a bandsaw or reciprocating saw, to cut along the perimeter of the shape. This can be a length process and often results in lower precision than desired as it depends on the operator of the saw to follow a line drawn to indicate the cutting path.
Endless chain cutters provide a significantly faster operation and a repeatable, predicable cutting path defined by the path taken by the chain. Example US patents include U.S. Pat. Nos. 3,833,311 and 3,884,280 both to Chailer and U.S. Pat. No. 5,853,269 to the present applicant. Each of these discloses a cutter having teeth carried by an endless loop chain and driven by an electric drill or similar drive mechanism. The precision of these cutters is acceptable, but leaves room for improvement. The cutting teeth are typically quite long relative to the size of the chain on which they are mounted. As a result, they can apply significant torque about the chain, allowing them to displace both inward and outward from the path of the chain. Further, the chain itself is not tightly constrained, and can wander significantly from the desired path.
Applicant""s previous patent, U.S. Pat. No. 5,853,269, addresses the precision problem by more tightly constraining the chain path by using channels rather than discrete guides and by providing a guide plate against which the teeth can bear. The teeth are then angled relative to their cutting direction to force them against the plate. This cutter has exhibited improved precision, but the teeth are still free to move outward when they are not engaged in the material. The result is that the teeth may start the cut outside of the desired cutting path and then track inward, leaving a somewhat ragged edge.
Prior endless chain cutters share several common problems. The first is that the large number of cutting teeth, simultaneously engaging the surface to be cut, apply significant rotational and linear forces to the cutter causing it to wander or jerk out of control. This is compounded by the fact that the operator can not grip the cutter itself to stabilize it, because of the teeth rapidly travelling around its perimeter. The typical solution has been to provide one or more drill points, commonly driven along with the cutting chain, which enter the material before the cutting teeth make contact. This alleviates much of problem, but the cutter is still free to tilt about these drill points, and where only one is used, is still free to rotate. The high forces generated by the cutter make it difficult for the operator to control the cutter and assure a smooth, even advancement of the cutter into the material. As a result, the prior cutters exhibit a tendency for the cutting teeth to jam in the material, suddenly stopping the cutter. The resultant kick back can damage the cutter, damage the surface being cut, and even injure the operator.
As mentioned briefly above, the prior endless chain cutters typically have the cutting teeth positioned to travel along the edge of the cutter. This exposes them to contact with the operator and with other elements in the environment. Were the operator to make contact with the rapidly moving teeth, injury would almost surely result. The teeth can also easily tangle clothing, hair, and even electrical cords.
The known prior cutters also typically generate a large amount of dust and debris in a very short amount of time. Where the material being cut is relatively hard, such as wood, this debris includes chips which can be flung outward by the teeth. Since the teeth are simultaneously cutting on all sides of the cutter, the debris is flung outward in all directions. This is both a safety hazard and a nuisance because the area which must be cleaned after using such a cutter can be extensive. Where the material is soft, such as plaster or drywall, the dust frequently becomes airborne, posing an inhalation threat and capable of traveling even greater distances before settling. In new construction, this may be less of a problem, but where the cutter is used in finished space, such as adding an outlet in an occupied room, such airborne dust, which may then settle on all of the furniture in a room, for example, is unacceptable.
There is a need for an improved endless chain cutter for cutting non-circular holes which offers improved precision in the cut. Further, the cutter should provide a more positive attachment to the surface being cut to avoid jamming and kickback. Ideally, the cutter would comprise a clutch or release mechanism to disconnect the drive in the event of a jam or other over torque situations. The cutter should incorporate a safety shield to enclose the cutting teeth and guard them from contact by the operator or with object in the surrounding environment. The cutter should also incorporate a dust shield to contain and preferably collect dust and debris generated by the cutter.
The present invention is directed to an improved apparatus for cutting no-circular holes in a single plunging operation. The apparatus uses an endless chain carrying multiple cutting teeth, the chain traveling around and through chain guides so that it follows a path corresponding to the shape to be cut. A drive mechanism connects the chain to a portable drill or other removable drive.
According to the invention there is provided an upper plate mounting the guides, drive mechanism, and drive chain and a lower plate serving to guide the cutting teeth. The cutting teeth are shaped to provide optimal cutting action in combination with an inward force to hold them against the lower plate. The upper plate rides on guide posts fixed to the lower plate, thus maintaining the alignment of the upper plate relative to the lower.
According to an aspect of the invention a means for temporarily mounting the cutter to the surface being cut is provided. Preferably this means comprises two or more screws which pass through the cutter and into the surface. Alternatively, the screws may pass through the guide posts, thus positively attaching the lower plate to the surface, or they may pass through the shield, below, attaching it to the surface.
According to another aspect of the invention a shield is provided which encloses the cutting teeth and drive chain, acting as a safety guard. This shield may also serve to contain dust and debris generated by the cutting teeth. The shield preferably is fitted with a hose coupling for the connection of a vacuum cleaner or dust collector hose so that the dust and debris can be evacuated. The shield is mounted so that it retracts, or floats, relative to the teeth so that it remains in position as the teeth plunge into the material.
Further in accordance with the invention the drive mechanism may incorporate a clutch so that it will slip when a jam or over torque condition occurs.
Still further in accordance with the invention, the lower edge of the shield may closely follow the outline of the lower plate, further maintaining the cutting teeth in a path closely following the edge of the lower plate.
The present invention offers significant advantages over the prior art devices. The use of guide posts positively maintains the alignment of the upper and lower plates, reducing the chance that the upper plate could cock relative to the lower, causing the teeth to jam. This is especially true where the mounting screws attach the lower plate firmly to the surface being cut.
The shield provides both a safety guard and a dust collection shroud around the cutter. The operator is protected from contact with the cutting teeth and chain and the generated dust and debris is collected as it is generated, preventing it from becoming airborne. When used with a quality vacuum or dust collector, the cutter may be used in an occupied space with no risk of contaminating nearby surfaces.
Should a jam occur, the clutch in the drive mechanism will reduce the amount of kickback force felt by the user and applied to the cutting teeth. This reduces both the chance of operator injury and the likelihood of damage to the cutter itself.
The above and other features and advantages of the present invention will become more clear from the detailed description of a specific illustrative embodiment thereof, presented below in conjunction with the accompanying drawings.